An internal illumination signboard is a signboard in which a light source is disposed such that the signboard itself emits light. Because of their excellent advertising effect, internal illumination signboards are used in various places.
In recent years, light-emitting diodes (LEDs) have been increasingly used as the light source of internal illumination signboards. Light-emitting diodes have excellent characteristics such as their small size, good power efficiency, capability of emitting light of brilliant colors, reduced risk of blowout, excellent initial drive characteristics, invulnerability to vibration, and invulnerability to repetitive switching between on and off.
The direction of the light emitted from a light-emitting diode is not controlled, and therefore, when the light emitted from the light-emitting diode is used as it is, the light is expanded and cannot efficiently illuminate the surface to be illuminated. For the purpose of controlling the direction of the light emitted from a light-emitting diode, a light source including the combination of a light-emitting diode and a lens has been proposed (see, for example, PTLs 1 and 2).
PTL 1 discloses a lens for a light-emitting diode that has an emission surface rotationally symmetrical (circularly symmetrical) about the optical axis of the light-emitting diode. PTL 2 discloses a lens for a light-emitting diode that has an emission surface of a substantially cylindrical form (which has a curvature on the first side thereof, and has no curvature on the second side orthogonal to the first side).